Clinical trials aimed at preventing the onset of disability in older persons will be done to test a variety of interventions in the future. For this work, it is critical to utilize disability outcomes that are reproducible, valid, and meaningful to clinicians as well as older persons themselves. The ability to walk 400 meters, as measured using an objective test of performance, has been proposed as an outcome that meets these criteria. In clinical trials, the inability to determine if an outcome event has occurred in a portion of the population is a serious problem that reduces the power of the study and results in loss of validity of the findings. This research will provide a method of imputing this outcome for clinical trial participants who are unable or refuse to come into the clinic to undergo the formal, objective 400 meter walk test. In this methodologic study, participants will be asked a series of self-report items and then receive the 400 meter walk test. A proxy will also be asked about the walking habits and abilities of the participant. Having this complete data, it will then be determined what self-report or proxy items, either alone or in combination, can be used to impute ability to walk 400 meters. A specific item, a subset of items, a scale, or expert adjudication of the whole battery of questions are potential ways to develop this imputation of actual ability to walk 400 m. This pilot study will make a number of contributions that will be of interest to the general geriatric community and will be critical to the development of a clinical trial of mobility disability prevention.